Image sensing devices made up of an array of laterally spaced sensors are well known and can take a variety of forms. The array can be viewed as being made up of a number of laterally offset regions, commonly referred to as pixels or sensing elements. The art has recognized that sensing advantages can be realized by forming a lens array having a convex lens surface for each pixel.
A lenticular array placed in registration with the device serves to focus the incident light upon the photodiodes. This has the effect of increasing the light gathering area of the photodiode and hence improving the signal.
The device has a lens supporting layer for directing light inwardly to a semiconductive light sensor. The fabrication of lenticular arrays is based upon using typical positive-working photoresists. A coating of the photoresist is exposed and developed to produce an array of resist "islands". The resist pattern is then flood exposed to bleach the remaining photochemistry and subsequently heated until the resist material flows enough to form a lens and yet not so much as to cause adjacent lenses to flow together. There are, however, several disadvantages to the use of conventional positive-working photoresists. The final resists absorb slightly in the blue portion of the spectrum and the absorption increases when the lenses are heated in air. The result of the increased absorption is a lower signal than could be obtained with more transparent lenses. To minimize this discoloration, the lenses are formed in a non-oxidizing atmosphere, such as nitrogen. The lens formation occurs in 10-60 seconds at a temperature of the resist below 180.degree. C. However, the desired packaging procedures of the finished devices typically require thermal treatment steps of up to 90 minutes at 150.degree. C. and about 60 seconds at 215.degree. C. Self-absorption of such lenses is further exaggerated by this temperature application over a period causing a yellowing or browning, see FIG. 4A. The use of these resists, therefore, necessitates a radically altered packaging process, with attendant complexities and disadvantages.